nir: Add a discard optimization pass

Many fragment shaders do a discard using relatively little information
but still put the discard fairly far down in the shader for no good
reason.  If the discard is moved higher up, we can possibly avoid doing
some or almost all of the work in the shader.  When this lets us skip
texturing operations, it's an especially high win.

One of the biggest offenders here is DXVK.  The D3D APIs have different
rules for discards than OpenGL and Vulkan.  One effective way (which is
what DXVK uses) to implement DX behavior on top of GL or Vulkan is to
wait until the very end of the shader to discard.  This ends up in the
pessimal case where we always do all of the work before discarding.
This pass helps some DXVK shaders significantly.

v2 (Jason Ekstrand):
 - Fix a couple of typos (Grazvydas, Ian)
 - Use the new nir_instr_move helper
 - Find all movable discards before moving anything so we don't
   accidentally re-order anything and break dependencies

v3 (Pierre-Eric): remove the call to nir_opt_conditional_discard based
on Daniel Schürmann comment.

v4 (Pierre-Eric):
 - handle demote intrinsics and drop derivatives_safe_after_discard
 - add early return if discards/demotes aren't used

v5 (Pierre-Eric):
 - use pass_flags instead of instr set (Daniel Schürmann)

v6 (Daniel Schürmann):
 - cleanup and fix pass_flags handling

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10522>

src/compiler/nir/meson.build

@@ -230,6 +230,7 @@ files_libnir = files(
   'nir_opt_loop_unroll.c',
   'nir_opt_memcpy.c',
   'nir_opt_move.c',
+  'nir_opt_move_discards_to_top.c',
   'nir_opt_offsets.c',
   'nir_opt_peephole_select.c',
   'nir_opt_rematerialize_compares.c',